Issue with portable Garmin unit

I have a 2003 325xi and my wife has a 2005 325ci. My portable Garmin unit works perfectly in my car however in the convertable the unit loses signal (satellites). It's never consistent. It may works fine for several hundred miles and other times lose signal every few miles. Both cars are pre-wired for phones. My dealer thinks there is phone wiring harness and can disconnect it.
Any thoughts????

I have a 2003 325xi and my wife has a 2005 325ci. My portable Garmin unit works perfectly in my car however in the convertable the unit loses signal (satellites). It's never consistent. It may works fine for several hundred miles and other times lose signal every few miles. Both cars are pre-wired for phones. My dealer thinks there is phone wiring harness and can disconnect it.
Any thoughts????

Its more likely a line-of-sight issue between the Garmin unit antenna and the sky. Make sure nothing is obstructing the pathway.

Also, do you have any after-market windshield tinting? That could also cause a signal decrease from the satellites (you need access to 3 for the Garmin to work).

Its more likely a line-of-sight issue between the Garmin unit antenna and the sky. Make sure nothing is obstructing the pathway.

Also, do you have any after-market windshield tinting? That could also cause a signal decrease from the satellites (you need access to 3 for the Garmin to work).

Thanks for the response. No window tint, thought it might be a problem with the roof, but the same situation occurs when the roof is down. I've even tried a separate Garmin antennae without success. It's definitely a signal from the car but don't know what. I've taken the unit out of the car about 20 ft and it works perfectly. As I walk towards the car (with engine running) it slowly loses satellite contact. I hope the car isn't haunted!!!!

You may be on to something with the TMPS. The convertable does have it, the other doesn't. Going into the shop tomorrow. We'll see.
Thanks for the response.
Picking up a 535i in Munich in October........I can't sleep!!!!